Ion Implantation :

This is much superior method of doping a semiconductor. In this technique
dopant atoms are ionized and accelarated through a chamber by the
application of high electric field. These ions bombard the semiconductor
surface and get implanted there and in this way the semiconductor is doped.

The ion implantation method has several advantages over diffusion that are
:

(1) The sources can be made of high purity by in situ mass spectrometric
technique.

(2) Deposition can be done by scanning the ion beam over a large surface
area.

(3) Dopant accuracy is very high due to in situ dopant concentration
monitoring techniques.

(4) By varying the ion dose doping concentration can be altered by orders
of magnitude.

(5) Almost total elimination of the lateral spreading effect which is a
huge problem for diffusion.

Though, there are many such advantages, there are also few short comings of
this ion implantation technique these are :

(1) High initial establishment cost of the ion implantation set up.

(2) Severe damage to the semiconductor surface due to high energy ion
bombardment which necessitates a post implantation annealing.

The annealing process is done typically at a temperature 350Â°-400Â°C for 10
to 30 min. and repairs these damages to a large extent by relocating
dislodged atoms to their proper lattice sites. Recently a technique known
as Rapid Thermal Annealing (RTA) is employed which is typically carried out
for a few seconds at high temperature of the order of 1000Â°C-1100Â°C.

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